The invention relates to a method for detecting leaks in a motor vehicle tank ventilation system.
A tank ventilation system consists of a fuel tank and an adsorption filter, generally an activated carbon filter, which are connected to one another by means of a ventilation conduit. The adsorption filter is connected to the atmosphere via an air conduit, which can be shut off by means of a shut-off valve, and is connected to the induction pipe of the internal combustion engine via a discharge conduit, which can be controlled by means of a regeneration valve. When the internal combustion engine is in operation and the shut-off valve and regeneration valve are open, the fuel vapors released from the fuel are induced into the induction pipe by the vacuum present therein. This process, furthermore, also disposes of the fuel vapors temporarily stored in the adsorption filter so that the adsorption filter is regenerated. When the vehicle is parked, the tank system is ventilated via the adsorption filter, in which the released fuel vapors are temporarily stored by chemical means.
As part of the stringent requirements of the American environmental authorities, measuring methods are required by which leaks in the fuel ventilation systems of vehicles can be detected.
Such a method is, for example, known from the German Patent Specification DE-A 40 03 751. In the method disclosed therein a pressure sensor is used to check whether a vacuum is maintained in the tank ventilation system when the shut-off valve is closed and the regeneration valve is open.
This method has the disadvantage that the vacuum generated in the tank ventilation system by the induction pipe vacuum is not constant but, rather, depends on various engine operating parameters. Consequently, a fault recognized when a predetermined pressure limit is exceeded can only be a rough determination. Small leaks cannot be recognized by this method.
In addition, a test method for liquid tanks is known from U.S. Pat. No. 4,442,702 in which a predetermined vacuum is generated in the tank and the pressure change is subsequently recorded with the valves closed. In a second step, a positive pressure is then applied to the tank and, with the valves likewise closed, the pressure change is again recorded. Conclusions are drawn regarding the leak-tightness of the tank system by comparing the slopes of the two pressure curves.
Although this method takes into account the fact that the pressure change in a closed tank system depends on the saturation vapor pressure of the liquid stored in the tank and therefore on parameters depending on the state of the liquid, the method has the disadvantage that additional equipment is necessary for generating a positive pressure in the tank system. This method is therefore too complicated and too expensive for use in a motor vehicle.
Accordingly it is the object of the present invention to provide a method for determining leaks in a motor vehicle tank ventilation system by which even relatively small leaks in a tank ventilation system can be reliably recognized in a simple manner.